1. Field of the Invention
This invention relates to a light measuring device having a multiple segmented photosensor with a plurality of computation formulae.
2. Description of the Related Art
Of the light measuring methods, the most common ones are the averaged overall light reading and the partial light reading. In the former, the various portions of the entire field of the photosensor are integrated to get an average of their different degrees of brightness. When the subject of principal interest is far dimmer than its environment, as in a back-lit scene, a proper exposure for the subject can be hardly obtained. The latter reading is able to isolate each of the portions of the scene, and, therefore, is suited for to determining the exposure based on the brightness of the subject. Because the light measuring area is of a fixed relative size to the entire area of the image frame, however, if the subject image occupies a small fraction of the photometric area, the determination of an exposure will be unavoidably influenced by the brightness of the background of the scene. Also, to desire a balanced exposure over the entire area of the image frame by mental summing and averaging technique, the partial light reading is not very useful. So, a camera having these two aspects of the light reading has been developed. But, since the choice of the better one of the two light measuring modes to a given scene depends on the insight and experience of the photographer, for the beginner, the camera became only harder to handle, rather troublesome, and was often mismanaged.
Recently, there has been proposed a light measuring device having a multiple segmented photosensor with an evaluation circuit receptive of the outputs of all the segments for obtaining a brightness distribution over the area of the image frame and responsive to this brightness distribution for determining an exposure suited to the scene. But, in this case also, since the relative size which the subject had to the image frame was ignored, the brightness of the subject was, similarly to the above-described partial light reading, not always primarily reflected is the exposure. Therefore, photographers could not take the light measurement with emphasis on a particular section of the scene. In more detail, as the size of the subject such as a person changes largely between landscape and portrait photography which are generally encountered, it was impossible to set that light measuring device in the mode suited to attain a best result of exposure of the subject.
Meanwhile, when the subject lies near or at the minimum distance, the back-lit situation of it is rarely encountered. For the subject distance of 1.5 to 3 meters, the possibility of encountering back-lit situations is high. On this account, a light measuring device capable of changing over between modes in response to object distance information has been developed as, for example, disclosed in Japanese Laid-Open Patent Applications No. SHO 56-102838 and No. SHO 58-12571.
Since, in such a proposal, however, the selection of the light measuring modes was made dependent only on the photographic distance, for, as the same subject at the same distance was shot, the focal length of the photographic lens varied in a wide range, even if the distance was the minimum, the possibility of occurrence of the back lighting in that scene would be very high. So, the last-named device had also a drawback that the exposure could not be based on the illumination of the subject.